In recent years, there have been provided various light emitting devices and illumination apparatuses using a solid state light emitting element such as a light emitting diode and an organic electroluminescence (EL) element as a light source. In Japanese Patent Application Publication No. 2011-9233, for example, there is disclosed an illumination apparatus in which a plurality of light emitting modules (light emitting devices) are connected in parallel to a constant current source.
In the light emitting module, a resistor, a transistor and one or more light emitting diodes are connected in series between a pair of main terminals, and a resistor for outputting connection information (information output resistor) is connected between a joint of the light emitting diode and one of the main terminals and an information output terminal. The information output resistors of the light emitting modules all are set to substantially the same resistance value.
The constant current source includes a pair of output terminals connected to the pair of main terminals of the light emitting module, an input terminal to which the connection information outputted from the information output terminal is inputted, a variable constant current source whose output current is variable, and a control unit for varying the current outputted from the variable constant current source according to the connection information.
With the technology disclosed in Japanese Patent Application Publication No. 2011-9233, the control unit of the constant current source determines the number of the light emitting modules connected between the main terminals on the basis of a voltage inputted to the input terminal, and varies the output current of the variable constant current source according to the number of the light emitting modules connected such that a predetermined current flows in each of the light emitting modules. Accordingly, despite changes in the number of the light emitting modules connected between the main terminals of the constant current source, a predetermined current (e.g., a rated current) can flow constantly in each light emitting module.
Also, there has been provided an illumination apparatus having a dimming function of varying light intensity and a toning function of changing light color. In this case, the light emitting module is composed of three types of light emitting diodes including, e.g., red light emitting diodes, green light emitting diodes and blue light emitting diodes, and luminous color can be changed by individually driving the light emitting diodes.
However, the solid state light emitting element such as a light emitting diode tends to have a large variation in light output due to a difference in the use environment or the production lot compared with other light sources such as fluorescent lamps. For example, in case of light emitting diodes, there is a variation in the magnitude of forward current flowing when the same forward voltage is applied, thereby resulting in variations in the light output.